1. Field of the Invention
The present invention relates to a DC/DC converter.
2. Description of the Related Art
In order to convert a DC voltage having a given level into a DC voltage having another level, a switching regulator (DC/DC converter) is employed. Such a switching regulator includes a switching transistor, a rectifier, an inductor, and an output capacitor. By adjusting the duty ratio of the on/off operation of the switching transistor, such an arrangement is configured to stabilize its output voltage to a desired level.
FIG. 1 is a circuit diagram which shows the configuration of a step-up switching regulator according to a comparison technique investigated by the present inventors. A step-up switching regulator 2r mainly includes an output circuit 102, a control circuit 100r, and resistors R1 and R2.
The output circuit 102 includes an inductor L1, a diode D1, and an output capacitor C1. The output circuit 102 has a topology employed in a typical step-up circuit, and accordingly, description thereof will be omitted. A switching transistor M1 is connected to the output circuit 102. By switching the switching transistor M1, such an arrangement is configured to boost the input voltage VIN so as to generate a boosted output voltage VOUT at the output capacitor C1. The output voltage VOUT is divided by the first resistor R1 and the second resistor R2, and the voltage thus divided is fed back to a feedback terminal (FB terminal) of the control circuit 100r. 
The control circuit 100r includes an error amplifier 10, a pulse modulator 12, a driver 14, and an overvoltage protection circuit 16, in addition to the switching transistor M1.
The error amplifier 10 is configured to amplify the difference between the feedback voltage VFB thus fed back to the FB terminal and a predetermined reference voltage VREF so as to generate an error voltage VERR. The pulse modulator 12 is configured to generate a pulse signal SP according to the error voltage VERR. The duty ratio of the pulse signal SP is adjusted such that the feedback voltage VFB matches the reference voltage VREF. As such a pulse modulator 12, a pulse width modulator (PWM) or a pulse frequency modulator (PFM) is employed. The driver 14 is configured to perform switching of the switching transistor M1 according to the pulse signal SP.
With such an arrangement, the output voltage VOUT of the switching regulator 2r is stabilized to the target level, i.e., VREF×(R1+R2)/2R.
Such a step-up switching regulator is configured to generate a high-level output voltage VOUT. Thus, in order to protect the circuit, an overvoltage protection circuit 16 is arranged. The overvoltage protection circuit 16 is configured to compare the feedback voltage VFB with a predetermined threshold voltage VTH, and to generate an overvoltage protection signal OVP which is asserted (e.g., set to high level) when VFB>VTH. That is to say, by means of the overvoltage protection circuit 16, such an arrangement is capable of detecting an overvoltage state. When the overvoltage protection signal OVP is asserted, the control circuit 100r is configured to perform a predetermined protection operation such as an operation for stopping the switching of the switching transistor M1.
The present inventors have investigated such a switching regulator 2r shown in FIG. 1, and have come to recognize the following problem.
External circuit components to be mounted on the control circuit 100r are electrically and mechanically mounted on a printed circuit board by means of solder. However, in some cases, such a circuit component could become disconnected from the printed circuit board due to various kinds of reasons.
If the rectifier diode D1 is disconnected from the control circuit 100r, no charge flows to the output capacitor C1, as a result of which the output voltage VOUT does not rise to the target level, i.e., remains at a low level. That is to say, the feedback voltage VFB remains lower than the reference voltage VREF, which instructs the pulse modulator 12 to generate the pulse signal SP having a maximum duty ratio.
If the switching transistor M1 is instructed to perform a switching operation at the maximum duty ratio due to disconnection of the diode D1, the electric potential VSW at a connection node (SW terminal) that connects the switching transistor M1 and the inductor L1 steadily rises and exceeds the breakdown voltage of the switching transistor M1. This has a danger of affecting the reliability of the switching transistor M1.
The above-described consideration is by no means within the scope of common and general knowledge in the field of the present invention. Furthermore, it can be said that the present applicant has been the first to arrive at this consideration.